early 14c., from Old French foaile "bundle of firewood," from Vulgar Latin legal term *focalia "right to demand material for making fire," neuter plural of Latin focalis "pertaining to a hearth," from focus "hearth" (see focus). Figurative use from 1570s.

(fy'əl) A substance that produces useful energy when it undergoes a chemical or nuclear reaction. Fuel such as coal, wood, oil, or gas provides energy when burned. Compounds in the body such as glucose are broken down into simpler compounds to provide energy for metabolic processes. Some radioactive substances, such as plutonium and tritium, provide energy by undergoing nuclear fission or fusion.

Almost every kind of combustible matter was used for fuel, such as the withered stalks of herbs (Matt. 6:30), thorns (Ps. 58:9; Eccl. 7:6), animal excrements (Ezek. 4:12-15; 15:4, 6; 21:32). Wood or charcoal is much used still in all the towns of Syria and Egypt. It is largely brought from the region of Hebron to Jerusalem. (See COAL.)